* src/descriptors/dr.h: new headers.
  * src/descriptors/Makefile.am: new descriptors.
  * misc/dr.xml, misc/test_dr.c: up to date descriptor definition and
    associated generated file.
  * src/descriptors/dr_0b.h, src/descriptors/dr_0b.c: MPEG2 "system
    clock" descriptor decoder and generator.
  * src/descriptors/dr_0c.h, src/descriptors/dr_0c.c: MPEG2 "multiplex
    buffer utilization" descriptor decoder and generator.
  * src/descriptors/dr_0d.h, src/descriptors/dr_0d.c: MPEG2 "copyright"
    descriptor decoder and generator.
  * src/descriptors/dr_0e.h, src/descriptors/dr_0e.c: MPEG2 "maximum
    bitrate" descriptor decoder and generator.
  * src/descriptors/dr_0f.h, src/descriptors/dr_0f.c: MPEG2 "private
    data indicator" descriptor decoder and generator.

Note: all the MPEG2 descriptors decoders and generators (except for wide
range data) have been tested with the "test_dr" program located in the
"misc" directory.